Comparing RF Circuit Material Processing Costs & Performance
Apr 1, 2011
John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm
Performance requirements typically guide the selection of a PCB material. Some applications may also be cost-sensitive, and require evaluation of the total costs of choosing a circuit material. This includes the cost of the material as well as costs associated with processing the material. For example, FR-4 is a low-cost material with minimal processing costs. However, its performance is also low relative to some higher-costing materials, such as PTFE- or hydrocarbon-based circuit materials, although these materials can have considerably different processing requirements and associated costs. By considering the costs of the material as well as its processing requirements, it’s possible to determine if “you get what you pay for” truly applies to circuit materials.
For example, Theta® halogen-free circuit materials from Rogers Corporation have a slight premium in cost to FR-4 materials, but with more stable dielectric constant over frequency and lower loss. Ideal for high-speed digital circuits and multilayer constructions, they feature high heat resistance for use in lead-free applications. Theta materials exhibit a dielectric constant of 3.90 at 1 GHz and 4.01 at 10 GHz (in the z-axis), with dissipation factor of 0.0080 at 1 GHz and 0.0118 at 10 GHz. While Theta laminate does not deliver the electrical performance of a PTFE-based laminate material such as Rogers RT/duroid® 6035HTC ceramic-filled PTFE composites, its material costs and processing costs are lower.
Circuits on Theta materials can be fabricated with standard processing methods, to minimize processing costs. The materials themselves are available with a wide range of laminate and prepreg thickness options for covering widespread applications, and they can be processed with the same methods used for FR-4 circuit materials, using standard pin or slotted tooling. Theta material requires no more than a standard chemical cleaning process with micro-etch for preparing board surfaces for the application of a liquid or dry film photoresist to “image” the desired circuitry; the copper surface can even be prepared for photoresist application by means of a basic mechanical scrub without damage to the material. The photoresist can be developed, etched, and stripped (DES) using any commercial chemical treatment. In many ways, Theta material circuit processing is just like that for FR-4 but Theta material features a z-axis coefficient of thermal expansion (CTE) that is 30% lower than FR-4, with significantly more stable dielectric constant over frequency and temperature than FR-4.
In contrast, PTFE-based circuit materials such as Rogers RT/duroid 6035HTC provide considerably better electrical performance than a hydrocarbon-based Theta material, although the cost of processing the material, and the material itself, is somewhat higher. RT/duroid 6035HTC is a ceramic-filled PTFE composite with low-profile, reverse-treated copper foil for excellent thermal dissipation, stability, and low loss. As a result, it is designed for much higher power-handling capability than Theta material.
The higher cost of the RT/duroid 6035HTC material itself and its processing, however, delivers a laminate with extremely stable dielectric constant of 3.50 in the z-axis at 10 GHz, with low dissipation factor of 0.0013 at 10 GHz and impressive thermal conductivity of 1.44 W/m/K for outstanding performance in high-power circuits.
Between Theta laminate and RT/duroid 6035HTC in terms of performance, material costs, and processing costs lies Rogers XT/duroid™ 8000 and RO4000® LoPro™ non-PTFE hydrocarbon circuit materials. XT/duroid 8000 is a halogen-free thermoplastic material designed for ease of processing. The material has a similar dielectric constant to that of RT/duroid 6035HTC: 3.23 in the z-axis at 10 GHz. It is resistant to the solvents and reagents typically used to process PCBs, and has a higher maximum operating temperature (MOT) than PTFE-based materials for handling higher-temperature environments. It is lower in material cost and in processing than RT/duroid 6035HTC, but with a higher 0.0035 dissipation factor (for increased loss) and much lower thermal conductivity for less power-handling capability. Also, Rogers RO4000 LoPro circuit material is based on a thermoset hydrocarbon resin system. Ideal for multilayer circuits, it features low-profile reverse-treated copper foil for low passive intermodulation (PIM) performance in RF/microwave circuits and excellent signal integrity in digital circuits. It is compatible with lead-free processing and designed for ease of fabrication to minimize processing costs. It cannot match the electrical performance of a PTFE-based circuit material like RT/duroid 6035HTC, but it is also less in material and processing costs. As can be seen from this sampling of Rogers’ materials, when considering a circuit material, tradeoffs in costs—both material and processing costs—also mean tradeoffs in performance.
Those attending the technical sessions at the IPC APEX EXPO in Las Vegas, NV (April 10-14, 2011, www.ipcapexexpo.org) can learn more about some of the performance, cost, and processing differences among circuit materials by attending John Coonrod’s presentation “Understanding when to use FR-4 laminates or high-frequency laminates.”